Wire bonders are used during semiconductor assembly and packaging for making electrical interconnections between electrical contact pads on a semiconductor die and a substrate, or between electrical contact pads on different semiconductor dies. Wire is fed from a wire spool containing bonding wire to a bonding tool such as a capillary for performing wire bonding.
A typical method used to bond or weld the wire to a connection pad is through a combination of heat, pressure and/or ultrasonic energy. It is a solid phase welding process, wherein the two metallic materials (the wire and the pad surface) are brought into intimate contact. Once the surfaces are in intimate contact, electron sharing or inter-diffusion of atoms takes place, resulting in the formation of a wire bond. The two main types of wire bond are ball bonds and wedge bonds.
In order to ensure consistent wire bonding performance, it is desirable to assess portability of wire bonding apparatus as well as the robustness of the bonding recipe. Typically, in order to do so, bonding parameters such as ball size, ball thickness, ball shear, intermetallic coverage, occurrence of ball lift, and occurrence of bond pad peeling are measured, and compared across wire bonders and/or recipes.
One widely used test of bond quality is the pull test. In a pull test, a series of bonded wires extending between respective pairs of bond pads are each pulled manually by tweezers or a hook in direction generally away from the bond pads. Dedicated wire pulling machines specifically designed for this task are also known. Bonded wires subjected to pulling in this fashion may fail according to one of a number of failure modes, including ball lift (the ball completely lifts away from its bond pad), ball neck failure (the neck between the wire and the ball fractures), heel breakage (a fracture at the heel of a wedge bond), weld lift (a wedge bond completely lifts away from its bond pad), or bond pad lift (aluminium metallization on the bond pad peels off to expose the substrate). The number of occurrences of each type of failure is determined by manual inspection of the bonds under a microscope, and used as an indicator of bond quality.
Existing pull test methods have a number of shortcomings. If wires are pulled manually, it is difficult, if not impossible, to compare test results since the pulling force may vary dramatically across different persons, and even within or between tests conducted by a single person. If wires are pulled using a dedicated wire pull machine, the test can be very time consuming, especially if it is desired to test a large number of wires. In either case, visual inspection and counting of bond failures is required, this being a very time consuming and error-prone task, especially for large sample sizes.
There remains a need, therefore, for a pull test method and system which overcomes or alleviates at least one of the foregoing difficulties, or which at least provides a useful alternative.